1. Field of the Invention
The present invention relates to a toner which is useful for electrophotography and the like, and to a method for manufacturing the toner.
2. Discussion of the Background
Currently, with the rapid digitalization in electrophotography, there is a steadily increasing need for a developer which can reproduce images having good image qualities. In addition, full color copiers and printers for the production of color images are in rapid increasing demand, because of the increase in demand for the preparation of color documents which are used for various presentations and for the reproduction of digital color images prepared by computers, digital cameras and scanners. Further, since computers are widely used in small offices and in home offices, attempts to miniaturize these image forming apparatuses and to reduce the costs thereof have been made. Furthermore, in order to save materials and avoid environmental pollution, recycling of these apparatuses and their supplies has been attempted. However, with the current state of the art, high-functional toners, which can produce good images, generally have poor reliability and, therefore, good images cannot be continuously produced over a long period of time.
The toners which are used in electrophotography are generally manufactured by the following method in which (1) a melted resin is mixed with additives such as a colorant, a charge controlling agent and the like, to disperse the additives in the resin; (2) the mixture is cooled; (3) the mixture is pulverized using high-speed air; and (4) the pulverized mixture is classified to obtain a toner having a proper particle diameter distribution.
In general, although the colorant is finely dispersed in the resin, the charge controlling agent is dispersed in the resin with relatively large particle diameter in comparison to the colorant. In addition, since the charge controlling agent is generally poorly compatibile with the resin, the charge controlling agent tends to easily separate from the resin upon pulverization. Therefore, the charge controlling agent often adheres to and contaminates the inside of the pulverizer. Further, the free charge controlling agent tends to transfer to the surface of the resultant toner, which results in a toner which does not have uniform charge properties. When the thus manufactured toner is used for a long time in an electrophotographic image forming apparatuses such as a copier or printer, the charge controlling agent easily releases from the toner and contaminates the developing units and other units of the apparatuses and carriers of the developers in the developing units; thereby resulting in deterioration of image quality.
The charge properties of a toner largely depend on the materials of the surface part of the toner. In other words, the charge controlling agent, which is dispersed inside the toner, has little affect on the charge properties of the toner. In addition, charge controlling agents are generally expensive. Therefore, a need exists for charge controlling agents which are preferably included on only the surface part of toners.
In attempting to solve these problems and in order to fulfill the need, Japanese Laid-Open Patent Publication No. 55-28032 discloses a magnetic toner in which fine particles consisting of a charge controlling agent are contacted with the surface of the toner at a high temperature, while a flow of air is supplied to fix the charge controlling agent to the toner surface. However, since the treatment is performed at a high temperature, a problem which occurs is that toner particles adhere to each other, resulting in formation of aggregates of the toner particles, and in addition a problem occurs in that the resultant toner has uneven charge properties, because the charge controlling agent cannot uniformly cover the surface of the toner. In addition, the fine particles cannot be entirely fixed on the surface of the toner and, therefore, the charge controlling agent is easily released from the toner by mechanical external forces.
Japanese Laid-Open Patent Publication No. 63-244056 discloses a toner which is manufactured by impacting colored particles with charge controlling particles and then passing the colored particles through a narrow space to fix the charge controlling particles thereon. However, a problem of the resultant toner is that the toner has uneven charge properties, because the covering of the charge controlling particles on the surface of each toner particle is not uniform and in addition the charge controlling particles fixed on the toner are easily released from the toner when the toner is mechanically mixed with a carrier, or rubbed with toner charging blades or toner layer regulating blades in developing units of image forming apparatuses.
In order to obtain images having good image qualities such as high resolution, attempts have been made to improve toners by preparing toners of relatively small average particle diameter and/or a narrow particle diameter distribution. However, toner particles manufactured by pulverization have irregular shapes, and when the toner particles are mixed with carriers and stirred in developing units, and/or rubbed with toner layer regulating blades or charging blades, the toner is further pulverized, thereby resulting in superfine toner particles and deterioration of image qualities. In addition, since the toner particles have an irregular shape, the toner exhibits poor fluidity and, therefore, the toner needs a large amount of a fluidizing agent. Another problem which occurs is that the filling factor of a toner bottle is relatively small, resulting in difficulties with miniaturization of toner bottles and image forming apparatuses.
Further image transfer processes for forming full color images in which full color images, which are formed on a photoconductor, are transferred to a transfer medium or a paper sheet are becoming more and more complex. Since the toner, which is manufactured by pulverization and, therefore, has an irregular shape, has poor transferability, a problem which tends to occur in that a portion of the transferred image is omitted, and when attempting to solve this problem, another problem occurs is that toner consumption increases.
Therefore, the need has increased for a toner which can produce images having good image qualities with small toner consumption, which results in decreases of running costs, by improving the transferability of the toner.
When a toner has good transferability, a toner cleaning device is not necessary for an image forming apparatus because almost all the toner images which form on a photoconductor or on an intermediate transfer medium are transferred to a transfer sheet and, therefore, miniaturization and cost reduction of image forming apparatus are possible and further the apparatus have an advantage in that disposal of waste toner is not needed.
In attempting to improve these drawbacks of toners having an irregular shape, a variety of methods have been proposed for manufacturing spherical toners.
For example, a particulate toner has been proposed which is manufactured by a suspension polymerization method in which oil drops, which include a monomer and toner property forming agents such as a colorant and which are dispersed in water are polymerized to form the particulate toner. The resultant toner is spherical and, therefore, the toner represents an improvement over the toner which has an irregular shape to some extent. However, it is difficult by the suspension polymerization method to obtain a toner having good transferability and cleaning properties by controlling its shape, for example, so as to have an intermediate shape between a spherical shape and an irregular shape.
In suspension polymerization, the conversion rate of monomer to polymer is preferably maximized and, therefore, it takes a long time to prepare the polymer. In addition, after the polymerization process and the following separation of the toner from water, the water and monomers which remain in the polymerized toner have to be removed from the toner. The removal of monomers is particularly difficult because monomers do not easily evaporate. On the other hand, toner particles generally melt and adhere to each other when dried at a temperature of 100.degree. C. or even lower. Therefore, toner particles are preferably dried at a temperature as low as possible, or vacuum drying is used to removing monomers from toner particles. Even when vacuum drying is performed at a relatively low temperature, a long time is required to remove monomer from toner particles, which results in increases in the manufacturing costs of the polymerized toner. In addition, when the toner particles are insufficiently dried, the toner particles adhere to each other when preserved at a relatively high temperature, resulting in the occurrence of blocking of the toner particles, or even if the toner particles do not adhere to each other, the monomer which remains in the toner particles seeps onto the surface of the toner particles, resulting in changes in the charge properties of toner and, therefore, the deterioration of image quality.
Further, since the particle diameter of polymerized toners is relatively small, a large amount of dispersant such as surfactant, inorganic particulate dispersant or water-soluble polymer protective colloid is needed for manufacturing the polymerized toners, which is different from general suspension polymerization. The dispersant tends to remain on the surface of the toner, resulting in deterioration of the charge properties of the resultant toner particularly under high humidity conditions. In order to remove these surfactants, the toner particles are preferably washed with a large amount of water for a long time. In this case, a large facility for treating waste water is needed, resulting in increases in manufacturing costs.
Furthermore, in suspension polymerization, which can be considered to be microbulk polymerization, it is difficult to control the average molecular weight and molecular weight distribution of the resultant polymer. These factors are important in the preparation of full color toners. Transparency and smoothness are important image qualities for full color images. When resins, which are used for the production of full color toner have too a high molecular weight, the color images produced by the resultant toners do not have the desired transparency and smoothness when the images are fixed by conventional heat energy. Low molecular weight polyesters, which have good fixability, cannot be prepared by condensation polymerization in water and, therefore, they are not available for suspension-polymerized toners.
Colorants such as pigments for use in the toner cannot be finely dispersed in monomers without dispersants. When a dispersant is used in such an amount that the resultant toner has good color properties, the charge properties of the toner are adversely affected to some extent by the dispersant. In addition, when the pigments are hydrophilic, the pigments move to the surface of the toner particles during polymerization, resulting in deterioration in the dispersion of the pigments. Accordingly, toners having good color properties and toner properties cannot be obtained. Thus the suspension polymerized spherical toners have a variety of problems as mentioned above.
Spherical toners can be obtained by another method in which a solution including a toner is formed as drops in water. In detail, at first toner materials such as a resin, a colorant and the like are dissolved or dispersed in an organic solvent to prepare a toner liquid, and then the toner liquid is mixed into water to prepare an emulsion including the liquid toner particles, and finally the water and the organic solvent are dried to prepare toner particles. In this method, spherical toner particles can be obtained, however, the shape of the toner particles cannot be controlled, which is the same drawback as that of the polymerized toner.
In addition, this method has a drawback in that the organic solvent which is used to dissolve the resin cannot be easily removed from the toner particles. The solvent tends to remain in the toner particles in a greater amount than the monomers in the case of the polymerized toners. Therefore, the toner particles are so tacky during drying that the particles adhere to each other, resulting in formation of aggregates of the toner particles. Even when an organic solvent having a low boiling point is used as the solvent, it takes a long time to remove (dry) the solvent from inside of the toner particles. If the drying is insufficient, the preservability and charge properties of the resultant toner are seriously affected. In addition, upon drying, voids tend to form as a result of evaporation of the solvent from the toner particles. Consequently, the resultant toner particles easily brake. The broken toner particles have too small a particle diameter to produce good images. Further, since a large amount of solvent is needed to manufacture the toner particles, an additional process and a facility for recycling the solvent are needed, resulting in increases in manufacturing costs.
In order to stabilize the drops of the liquid toner particles, a dispersant is needed. When a dispersant is used, a problem tends to occur which is similar to the problem of polymerized toner and, therefore, an additional process and a facility for washing the resultant toner particles are needed, resulting in increases of manufacturing costs. When self-emulsifying resins are used as the toner resin, it is possible to reduce the amount of the dispersant added or to remove the dispersant from the toner liquid. However, the self-emulsifying resins tend to locate on the surface of the toner particles and, therefore, the charge properties of the toner deteriorate.
The resin which is used in this toner manufacturing method is limited to resins which can be dissolved in organic solvents which are not dissolvable in water, although the number of the resins is not as small as the number of resins which are used in the polymerized toner.
In this toner manufacturing method, it is often difficult to disperse a colorant in a resin solution, because the colorant such as a pigment does not always adsorb the resin. When a dispersant is used, a similar problem to that of the polymerized toner occurs and also the charge properties of the resultant toner deteriorates.
In Japanese Laid-Open Patent Publication No. 10-142838, the present inventor discloses a toner for electrophotography which contains very little volatile organic component and a free fluidizing agent and whose shape can be controlled. A method is also disclosed for manufacturing the toner in which the primary particles of the toner are dispersed in a liquid which does not dissolve the resin and in which the particles are heated for an extremely short time to control the toner particles. In addition, the publication discloses that by using this method a charge controlling agent can be securely fixed on the surface of the toner particles, resulting in prevention of deterioration of the charge properties of the resultant toner. The present invention further improves the release of the charge controlling agent from the toner particles, which causes contamination of carriers and developing units used for producing images.